Circularly polarized antenna

ABSTRACT

An antenna for launching and receiving circularly and eliptically polarized electromagnetic waves includes a pair of conductive elements. Each of the elements has a horizontal portion and a continuing vertical portion. The horizontal portions of the first and second elements are arranged to form a first center fed dipole oriented in the horizontal plane and the vertical portions of the first and second elements are arranged to form a second center fed dipole oriented in the vertical plane. The center feed point of the first dipole is spaced from the center feed point of the second dipole so that the phase center of the waves associated with the two dipoles are at phase quadrature.

[11] 3,765,023 [4 1 Oct. 9, 1973 United States Patent v Ben-Dov [57]ABSTRACT An antenna for launching and receiving circularly andeliptically polarized electromagnetic waves includes a pair ofconductive elements. Each of the elements has a horizontal portion and acontinuing vertical portion. The horizontal portions of the first andsecond elements are arranged to form a first center fed dipole orientedin the horizontal plane and the vertical portions of the first andsecond elements are arranged to V. W w Nm AmN D m fm VIZ o 7 ne Lmm. CHQP Y W1 dC m0R&w. Mum o Umm N Cn L T mmi CmAFA form a second center feddipole oriented in the vertical plane. The center feed point of thefirst dipole is spaced from the center feed point of the second dipoleso that the phase centerof the wavesassociated with the two dipoles areat phase quadrature.

06 00 07 0 N1 32 ,3 4 .84 3 93 Q 7 7 9H7 W 3 H 4 3 3 4 3 N h n c u S L ml s k UhF U 555 [56] References Cited UNITED STATES PATENTS 6 Claims, 4Drawing Figures Spanos et Primary Examiner-Eli Lieberman Att0rney-EdwardJ. Norton et a1.

CIRCULARLY POLARIZED ANTENNA BACKGROUND OF THE INVENTION This inventionrelates to an improvement in antennas of the type which radiate and/orreceive circularly polarized electromagnetic waves.

In connection with radio and television broadcasting, it is known thatto afford satisfactory reception regardless of the orientation of thereceiving antenna with respect to the transmitting antenna, a circularlypolarized transmitting antenna is desirable. It is also desirable thatboth the vertical polarization and the horizontal polarization radiationpatterns be equal. Further, it is desirable that the azimuthal orhorizontal radiation patterns have a substantially uniform beam widthover the required bandwidth of about 120 so that, when three suchantennas are mounted to a triangular tower which is already in use ofother transmission purposes. (for example television VHF broadcast), asubstantially uniform omni-directional pattern can be achieved. If thisomni-directional pattern is not achieved, then the reception about thecoverage area of the radio broadcasting station is not uniform and henceusually undesirable.

Briefly, these desirable patterns are provided by a circularly polarizedantenna of the present invention which is operable over a givenfrequency band. The antenna includes a first conductive element having afirst and a continuing second portion spaced forward of a reflectingsurface. A second conductive element having a first and a continuingsecond portion is spaced forward of the reflecting surface. The firstportions of the first and second elements are symmetrically disposedrelative to each other and extend in a common plane which isperpendicular to the reflecting surface in a manner to-form a firstcenter fed dipole fed at the free ends of the first portions. The secondportion of the first element spaced from the reflecting surface extendsparallel to the reflecting surface and perpendicular to and above thecommon plane of the two first portions. The second portion of the secondelement extends parallel to the reflecting surfaceand is perpendicularto and below the common plane to form with the second portion of thefirst-element a second center feddipole orthogonally oriented to thefirst dipole. The junction FIG. 4 illustrates a typical horizontalpattern for an antenna system arrangement as shown in FIG. 3.

Referring to FIG. 1, the antenna 10 is customarily mounted on anelevating support structure or tower not shown. The antenna 10 includesa pair of conductive elements 31 and spaced from a screen reflector 15.The screen reflector 15 is made up of a plurality of verticallyextending rods 17 coupled to a plurality of horizontally extending rods19. A pair of horizontally extending hollow support arms 21 and 22 arecoupled by clamps 23 and 24 to a center rod 19a of the horizontallyextending reflector rods 19. The arms 21 and 22 extend perpendicular thescreen reflector 15 with the ends 25 and 27 remote from the reflector 15being the feed points or feed terminals for the elements 31 and 45 ofthe antenna 10. The element 31 is connected to support arm 21 at point250 near end 25. The element 45 is connected to support arm 22 atpoint'27a near end 27.

The first antenna element 31 comprises a first portion 33 orientedhorizontal (parallel to the ground) and has a second portion 35perpendicular to the first portion and extending vertically. The portion33 includes a first section 39 and a second section 38 joined togetherat point 37. The section 39 extends from a point 25a close to the end 25of arm 21 at an angle of about 30 away from arm 21 to the point 37. Thesection 39 is approximately one-quarter wavelength long at the centeroperating frequency of the antenna. The section 38'is at right angles tosection 39 and extends to point 43 approximately A: wavelength at thecenter operating frequency of the antenna from point 37 to point 43. Thesections 38 and 39 of portion 33 lie in a common plane which issubstantially perpendicular to the reflector 15. The overall length ofconductive portion 33 is about three-eights ofa wavelength at the centeroperating frequency of the antenna. The straight line length in spacebetween points 43 and 25 is approximately /:t wavelength at the centeroperating frequency of the antenna. a I 'The portion 35 is substantiallyvertical and perpendicular to the plane formed by. portion 33 and isparallel to and spaced from the vertical rods 17 of reflector of thefirst and second portions of the first element and the junction of thefirst and second portions of-the second element form the feed point ofthe second dipole. The straight line spacing'between the feed points ofthe first and second center fed dipoles is madeso that the phase centerof the waves associated with the first dipole is at phase quadraturewith the phase center of the waves associated with the second dipole.The RF electromagnetic signal energy at the operating frequency is fedto the first element and the second element to establish a standing waveon the antenna at the given frequency band.

DETAILED DESCRIPTION A more detailed descriptionfollows in conjunctionwith the accompanying drawings in which:

FIG. 1 is a perspective view showing a circularly polarized antennamounted forward ofa reflector screen.

15. The vertical portion 35 is approximately threeeighths .ofawavelength long at the center operating frequency of the antenna andextends above the plane formed by portion 33.

The antenna 10 includes the second antenna element 45 having ahorizontally extending first portion 47 symmetrically disposed relativeto the first portion 33 and a vertically extending second portion 51.The first portion 47 and the second portion 51 are joined at point 53.The first portion 47 includes substantially horizontally extending firstsection 49 which extends in the right angle therewith..The section 55extends toward the arms 21 and'22- for a length of about an eight of awavelength at the center operating frequency to point 53. Thehorizontally extending sections 39 and 49 are approximately at an angleof 60 relative to each other.-

The vertical portion 51 extends parallel to the refleeting surface 15and the rods 17 and extends for a length of about three-eighths of awavelength long at the center operating frequency of the antenna frompoint 53. The second portion 51 extends below the plane formed by firstportions 33 and 47. The vertical portions 35 and 51 are offset fromalignment with each other by a distance D equal to about one-tenth of awavelength at the center operating frequency to achieve a verticalpolarization beam width of 120. By placing the horizontally extendingmembers as shown with sections 39 and 49 at an angle of about 60 withrespect to each other and forward ofreflecting surface 15, a horizontalpolarization beam width of about 120 is obtained.

The antenna is coupled to a coaxial transmission line that extendsthrough the inside of hollow arm 21. The

outer conductor 28a of this coaxial transmission line is connected toend 25 of arm 21 and consequently to one end of first antenna element31. The inner conductor 28 of the coaxial transmission line is connectedto end 27 of arm 22 and consequently to one end of the secnd element 45.The opposite end of this coaxial transmission line is coupled to anappropriate source or load, not shown. It has been found that by makingthe spacing between the feed points 43, 53 of the vertical portions 35-,51 and the feed points 25, 27 of the horizontal portions 33 and 47approximately one-third of a wavelength at the center operatingfrequency of the antenna, the phase center of the waves associated withthe vertical dipole and the phase center of the waves associated withthe horizontal dipole are effectively at about phase quadrature (90phase difference) and circular polarization is achieved.

To provide improved impedance matching for a relatively low impedancetransmission line such as 50 ohms, for example, and for adjusting thephase centers of the waves associated with the vertical dipole so as tomore closely achieve effective phase quadrature between these waves, apair of matching elements 61 and 65 are provided. The first matchingelement 61 is spaced by elements 63 and 64 a distance of about 8 inchesparallel to section 39 of portion 33. Spacer element 63 is coupled toarm 21 at end 25 and to element 61. Spacer element 64 is coupled tosection 39 and ele- Y ment 61. Element 61 is approximately 19 incheslong.

The second matching element 65 is spaced parallel to section 49 ofportion 47 by elements 67 and 68. Spacer element 67 is coupled betweenarm 22 at end 27 and element 65. Spacer element 68 is coupled betweensection 49 and element 65. The element 65 is about 19 inches long andspaced about eight inches from element 49. In the above describedarrangement the desired patterns were achieved for the dimensions givenwhen operating at 90 MHz.

Added support for the vertical elements 35 and 51 can'be provided byconnecting a support 69 between rods 19 of reflector and portion 35 atpoint 71 onequarter wavelength away at the operating frequency of theantenna from tip 73 of portion 35. The vertical element 51 isadditionally supported by support 75 coupled between the rods 19 ofreflector l5 and point 77 of portion 51. Point 77 of portion 51 islocated onequarter of an operating frequency wavelength from the tip 78of portion 51.

Referring to FIG. 2, there is illustrated a typical horizontal patternwith relative field strengths for an antenna of the type described abovein connection with FIG. 1. It can be noted that a substantially uniformbeam width pattern of 120 is provided.

Referring to FIG. 3, a tower 81 is described having sides 83, 85 and 87.With three such circularly polarized antennas and their associatedreflectors 91, 93 and 95 of the type described in FIG. I mounted to thesides of the tower as shown in FIG. 3, a substantially uniformomni-directional pattern as shown in FIG. 4 is provided.

What is claimed is:

l. A circularly polarized antenna operable over a given frequency bandcomprising:

a reflecting surface,

a first conductive element having a first portion and an electricallycontinuous second portion spaced forward of said reflecting surface,

a second conductive element having a first portion and an electricallycontinuous second portion spaced forward of said reflecting surface,

means coupled to a terminating end of each of said conductive elementsfor feeding RF electromagnetic signal waves between said firstconductive element and said second conductive element at a terminal endthereof to establish a standing wave on said conductive elements at saidgiven frequency,

said first portion of said first element and said first portion of saidsecond element being symmetrically disposed relative to each other andextending in a common plane to form a first dipole spaced from saidreflecting surface,

said second portion of said first conductive element extending parallelto said reflecting surface and perpendicular to and above said commonplane, said second portion of said second conductive element extendingparallel to said reflecting surface and perpendicular to and below saidcommon plane to form with the second portion of said first element asecond dipole orthogonally oriented to said first dipole. said firstconductive element and said second conductive element being furtherarranged so that the straight line spacing between the junctions of thefirst portion and second portions of said conductive elements and saidterminating ends is sufficient to efiectively accomplish quadraturephasing between the phase centers of the waves at said first dipole andsaid second dipole.

2. The combination as claimed in claiml wherein said first portion ofsaid first conductive element and said first portion of said secondconductive element near the feed terminating end of these elements areat an angle with respect to each other of about 60.

3. The combination as claimed in claim 1 wherein said first and secondportions of said first and second conductive elements are each aboutthree-eighths of a wavelength long at a frequency within said givenfrequency band.

4. The combination as claimed in claim 3 wherein the second portion ofsaid first conductive element is offset from alignment with the secondportion of the second conductive element by a distance d M10, where )tis a wavelength at a frequency within said frequency band.

5. The combination as caimed in claim 1 wherein each of said firstportions has two sections at right angles with respect to each other.

6. The combination as claimed in claim Swherein a first section of saidtwo sections is approximately onequarter wavelength at said operatingfrequency.

1. A circularly polarized antenna operable oVer a given frequency bandcomprising: a reflecting surface, a first conductive element having afirst portion and an electrically continuous second portion spacedforward of said reflecting surface, a second conductive element having afirst portion and an electrically continuous second portion spacedforward of said reflecting surface, means coupled to a terminating endof each of said conductive elements for feeding RF electromagneticsignal waves between said first conductive element and said secondconductive element at a terminal end thereof to establish a standingwave on said conductive elements at said given frequency, said firstportion of said first element and said first portion of said secondelement being symmetrically disposed relative to each other andextending in a common plane to form a first dipole spaced from saidreflecting surface, said second portion of said first conductive elementextending parallel to said reflecting surface and perpendicular to andabove said common plane, said second portion of said second conductiveelement extending parallel to said reflecting surface and perpendicularto and below said common plane to form with the second portion of saidfirst element a second dipole orthogonally oriented to said firstdipole, said first conductive element and said second conductive elementbeing further arranged so that the straight line spacing between thejunctions of the first portion and second portions of said conductiveelements and said terminating ends is sufficient to effectivelyaccomplish quadrature phasing between the phase centers of the waves atsaid first dipole and said second dipole.
 2. The combination as claimedin claim 1 wherein said first portion of said first conductive elementand said first portion of said second conductive element near the feedterminating end of these elements are at an angle with respect to eachother of about 60*.
 3. The combination as claimed in claim 1 whereinsaid first and second portions of said first and second conductiveelements are each about three-eighths of a wavelength long at afrequency within said given frequency band.
 4. The combination asclaimed in claim 3 wherein the second portion of said first conductiveelement is offset from alignment with the second portion of the secondconductive element by a distance d lambda /10, where lambda is awavelength at a frequency within said frequency band.
 5. The combinationas claimed in claim 1 wherein each of said first portions has twosections at right angles with respect to each other.
 6. The combinationas claimed in claim 5 wherein a first section of said two sections isapproximately one-quarter wavelength at said operating frequency.